1. Field of Invention
The present invention relates to an airway adapter for a non-dispersive infrared gas analyzer in which an infrared ray passes through apertures provided at opposite side wall portions of a flow tube, and an infrared ray detector measures an amount of the infrared ray which is absorbed and attenuated by gas flowing within the flow tube thereby to measure a concentration of the gas. In particular, the present invention relates to an airway adapter for a non-dispersive infrared gas analyzer which is provided between a patient and a respirator or an anesthesia machine and which measures the concentrations of various gases within a respiratory gas.
2. Related Art
When a mechanical ventilator assists a patient in breathing, an airway adapter is attached to a flow path between the patient and a mechanical ventilator and a concentration of carbon dioxide within the respiratory gas is measured. FIG. 12 is a perspective view showing an example of the outward appearance of a generally known airway adapter. In FIG. 12, the center portion of a cylindrical flow tube 1 along the axial direction thereof is formed in a rectangular tubular shape in section. Circular aperture portions 2 formed by openings for passing an infrared ray therethrough are provided at opposite positions of both side walls of the flow tube, respectively. Transparent windows 3 formed by transparent films configured in a circular plate shape are hermetically attached to the aperture portions 2, respectively.
As the transparent window 3, there has been employed sapphire or a plastic sheet formed by a single sheet on which an anti-fogging layer is formed. As the plastic sheet, there has been employed a polyester sheet (hereinafter called a PET sheet) 5 on which a anti-fogging layer 4 can be easily formed by coating or deposition, as shown in FIG. 13.
However, when the sapphire is employed as the material of the transparent window 3, there arises a problem that the cost of the transparent window is expensive. Further, in this case, disadvantageously it is required to heat the transparent window by using a heater to prevent a condensation of water in the respiratory gas on the surface of the sapphire.
The PET sheet 5 on which the anti-fogging layer 4 is formed may be used as the transparent window 3 in place of the expansive sapphire. In this case, the cost for manufacturing the airway adapter can be reduced. Further, since the anti-fogging layer is hydrophilic, a thin water layer instead of water drops is formed on the surface of the transparent window 3, whereby an infrared ray transmits through the transparent windows 3 without being scattered and so the transparent windows do not fog. Thus, the PET sheet has such an advantage that, since it is not necessary to heat the PET sheet by using a heater, the configuration of the analyzer can be simplified and the power consumption of the analyzer can be reduced. However, the PET sheet 5 is required to be made thinner in order to improve the transmittance of the infrared ray since it absorbs infrared rays. If the PET sheet is made thinner, the PET sheet is likely damaged by the heat or mechanical impact. Further, if the PET sheet 5 is made thinner, there arises a problem that, in the case where a water layer is formed on the surface of the anti-fogging layer 4, the measured value of the gas concentration may contain an error due to the optical interference.
Polypropylene and polyethylene materials have good transmittance for an infrared ray. The transparent window 3 can be made sufficiently thick by using such material. However, in this case, the anti-fogging layer 4 can not be formed on the surface of the transparent window since each of the polypropylene and polyethylene is not good in adhesive property.